split: Further option parsing improvements.

• Whenever possible, use strtonum() to parse numeric arguments.
• Improve usefulness and consistency of error messages.
• While here, fix some type and style issues.

Sponsored by: Klara, Inc.
Reviewed by: christos, kevans
Differential Revision: https://reviews.freebsd.org/D42010

(cherry picked from commit d7399551b02861b910b15c7b7a78a2835644aa45)

split: Improve GNU compatibility.

• Like GNU split, turn autoextend back on if given -a0.
• Add a test case that verifies that -a<non-zero> turns autoextend off.
• Add a test case that verifies that -a0 turns autoextend back on.

Sponsored by: Klara, Inc.
Reviewed by: christos, kevans
Differential Revision: https://reviews.freebsd.org/D42011

(cherry picked from commit d9d69a6f6fc1f8d80df9ce6493e8ab9388781615)